X-ray scanning has been used to identify explosive materials, such as TNT, and wires of electronic control, timing, and/or detonation devices for explosive devices in suspect objects, for example. X-ray scanning may also be used to identify high atomic number material that may be special nuclear materials, such as uranium and plutonium, or shielding for such materials, such as tungsten and lead. X-ray scanning may also identify explosive devices that could be used to disperse radioactive, chemical, or biological materials.
Radiation having peak energies of about 0.5 MeV and higher typically comprise a particle accelerator, such as a linear radiofrequency (“RF”) particle accelerator, to accelerate charged particles, and a source of charged particles, such as an electron gun, to inject charged particles into the accelerator. The linear accelerator may comprise a series of linearly arranged, electromagnetically coupled resonant cavities in which standing or traveling electromagnetic waves for accelerating the charged particles are supported. The charged particles injected into the resonant cavities are accelerated up to a desired energy and directed toward a conversion target to produce radiation. Where the accelerated charged particles are electrons and the target is a heavy material, such as tungsten, Bremsstrahlung or X-ray radiation is generated. Electrons accelerated to a nominal energy of 1 MeV and impacting tungsten, will cause generation of X-ray radiation having a peak energy of 1 MeV, for example.
A microwave (RF) power source provides RF power to the cavities of the accelerator. The microwave source may be an oscillating microwave power tube, such as a magnetron, or an amplifying microwave power tube, such as a klystron. The microwave sources are powered by modulators, which generate high electric power pulses having peak electric powers of from 1 MW to 10 MW, and average powers of from 1 kW to 40 kW, for example.
Typical MeV radiation sources weigh several tons. Once set up at a location for scanning, they are not readily moved. Portable MeV radiation sources are known, which can be moved by truck or forklift, for example. They may be more readily moved to different locations.
One example of a portable MeV radiation source is a Mini-Linatron, which was available from Varian Associates, Palo Alto, Calif. As described in literature from Varian Associates, the Mini-Linatron comprised an X-ray head, a power module, a control case, and a modulator module that were connectable by transmission lines, cables and hoses. The X-ray head, which is said to have weighed from 100 pounds (45 kg) including a 2 MV-6 MV accelerator, and 300 pounds (136 kg) including a 9 MV accelerator, also contained an ion chamber and a collimator. The power module, which is said to have weighed 300 pounds (136 kg), contained a magnetron, a pulse transformer, and other RF components. The modulator module, which weighed 300 pounds (136 kg), is also said to have contained a pulse modulator, an electronic line-type chassis, and a power supply. The control case is said to have weighed 11 pounds (5 kg). A Mini-Linatron including a 2 MV accelerator therefore weighed about 711 pounds (323 kg). “MINI Field Portable X-Ray Equipment,” Varian Associates, October 1997.
Another modular high energy source for mobile and fixed installations, available from Varian Medical Systems, Inc., Palo Alto, Calif. (“Varian”), is the Linatron®-M™. An X-ray head module including a 3 MV M3 Linatron® accelerator and RF unit, which includes a magnetron and pulse transformer, is said to weigh 1,950 pounds (886.3 kg). “Linatron®-M™ Modular high every radiation source,” Varian Medical Systems, Inc., September 2007.
Another portable system which was available from Varian is the Linatron-MP, in which an X-ray head module including a 4 MV accelerator weighs 150 pounds (68 kg), a modulator cabinet weighs 685 pounds (311 kg), and an RF unit weighs 340 pounds (155 kg). “VARIAN'S LINATRON-MP: THE PORTABLE SYSTEM” for Field Radiography,” Varian Medical Systems Technology, Inc., 2003.
Russell G. Schonberg describes a 4 MeV traveling wave accelerator packaged with a 9.3 GHz magnetron r.f. source and a pulse transformer, weighing about 190 pounds (86 kg), in “A History of the Portable Linear Accelerator.” Schonberg states that “the total weight was marginal for two people at 190 pounds . . . ” Schonberg does not identify the weight of the modulator and power supplies, which, as described above, typically weigh many hundreds of pounds. “The History of the Portable Linear Accelerator”, Russell G. Schonberg, The American Association of Physicists in Medicine, Annual Meeting, 2001.